1. Field of the Invention
The present invention relates to a projection display apparatus, and more particularly to a projection lens of a projection display apparatus which not only has a wide field of view of 60xc2x0 or more to reduce the thickness of a projection TV (or monitor), to which the projection lens is applied, but also has a long back focal length to allow an easy arrangement of color separating/synthesizing optical elements while having a telecentric structure to achieve an enhancement in the performance of the color separating/synthesizing optical elements and a reduction in longitudinal chromatic aberration.
2. Description of the Prior Art
Generally, modern persons have an increased tendency to enjoy private leisure and recreation. For instance, they have an increased tendency to see movies or other pictures in their private spaces. In pace with such an increased tendency, active research and development have been made to provide image display devices having a screen of an increased size.
By virtue of such research and development coping with the tendency to provide a screen with an increased size, data projectors, projection TVs, and projection monitors have been proposed, which utilize a projection technique incorporating the concept of projectors. These devices use a projection display apparatus.
The tendency to provide a screen with an increased size has also resulted in active development of rear projection type TVs (or monitors) that utilize a projection method to constitute a display system. Such rear projection type TVs (or monitors) require an enhanced picture quality to cope with an increased screen size, a minimized system size including a minimized thickness, an increase in the minimum number of pixels, an increased amount of light, and an increased uniformity of illumination.
Now, the configuration of the above mentioned projection display apparatus would be described in brief with reference to FIG. 1.
In FIG. 1, the reference number 11 denotes a light source, which is an ultra-high pressure mercury lamp, the reference number 12 denotes a mirror, and the reference number 13 denotes illumination optics. Also, the reference number 21 denotes a polarized beam splitter (PBS), 22 a reflective light valve (imager), 31 a projection lens, and 32 a screen.
The ultra-high pressure mercury lamp 11 is a light source for the projection display apparatus. The brightness of the projection display apparatus is determined by the amount of light emitted from the ultra-high pressure mercury lamp 11.
After being reflected by the mirror 12, light from the ultra-high pressure mercury lamp 11 passes through the illumination optics 13. This illumination optics 13 serves to render the light from the ultra-high pressure mercury lamp 11 to be a uniform and substantially parallel light so as to allow the light to be focused onto the reflective light valve 22 at a maximum efficiency. For the illumination optics 13, a light tunnel, light pipe, or fly-eye lens is used. The reason why the light pipe is used that the light emitted from the lamp exhibits a non-uniform intensity such that it exhibits a strong intensity near its optical axis while exhibiting a gradually reduced intensity as it is spaced away from the optical axis. When such a light, which emerges from an LCD after being reflected by that LCD, is projected onto a screen, it forms an image exhibiting a non-uniform brightness. For this reason, a light pipe is used to make light with a maximum uniformity emitted from a lamp have a non-uniform intensity.
The light emerging from the illumination optics 13 is incident onto the reflective light valve 22 via the PBS 21. The incident light is modulated by an output signal from the reflective light valve 22, and then reflected by a back plain. Finally, the light is projected in an enlarged state onto the screen 32 via the projection lens 31.
The projection lens 31 should have a very short projection length in order to minimize the thickness of a projection TV or monitor to which the projection lens 31 is applied. In other words, it is necessary to use a projection lens having a large field of view. Furthermore, for the projection lens 31, it is necessary to use a high performance projection lens having a high-resolution power to cope with an increase in the minimum number of pixels and a reduction in the effective size of the light valve. Although the uniformity in the amount of light and illumination in the projection system mainly depends on the performance of the illumination system used in the projection system, it is necessary to use a projection lens having a relatively low F-number (f/3.5 or less) and a high marginal light amount ratio (80% or more).
In the above mentioned projection display apparatus, a polarizing prism or dichroic filter should be arranged between the projection lens and the light valve in order to change respective axes of the optical path of the illumination system and the optical path of the projection lens 31 or to conduct a color separation/synthesis. By virtue of the arrangement of such an optical element, the projection lens must have a sufficiently long back focal length.
Furthermore, the polarizing prism or dichroic filter is provided with an optical coating in order to obtain a desired performance thereof. However, such an optical coating causes a considerable variation in the performance of the prism depending on the angle of incident light with respect to the plane of incidence.
For this reason, main beam of respective fields incident onto such an optical element should be incident vertically onto the light valve so as to avoid degradation in the optical performance at each field. To meet such a condition, the projection lens must have a telecentric structure.
The projection lens 31 exhibits a longitudinal chromatic aberration in the form of a misconvergence of red (R), green (G), and blue (B) colors. When such a misconvergence increases, one white line is displayed in the form of three separate R, G, and B lines. In other words, an increased misconvergence results in degradation in picture quality. Where the above mentioned projection display apparatus is used as a monitor adapted to mainly display characters, such a misconvergence causes a more severe problem. Accordingly, the projection lens should be implemented to exhibit a very small longitudinal chromatic aberration.
However, there is not known any lens capable of meeting performance requirements for the projection lens of the above mentioned projection display apparatus.
Although it is possible to easily implement a projection lens meeting one or a part of performance requirements for the projection display apparatus, it is difficult to implement a projection lens meeting all the given performance requirements. In particular, it is more difficult to implement a telecentric projection lens having a wide field of view of 60xc2x0 or more and a long back focal length while meeting diverse performance requirements mentioned above.
Therefore, the present invention has been made in view of the above mentioned problems, and an object of the invention is to provide a projection lens of a projection display apparatus which is of a telecentric structure having a wide field of view of 60xc2x0 or more and a long back focal length while meeting performance requirements for a projection system of a large screen, minimum thickness, and high picture quality.
Another object of the invention is to provide a projection lens of a projection display apparatus which not only has a wide field of view of 60xc2x0 or more to reduce the thickness of a projection TV (or monitor), to which the projection lens is applied, but also has a long back focal length to allow an easy arrangement of color separating/synthesizing optical elements while having a telecentric structure to achieve an enhancement in the performance of the color separating/synthesizing optical elements and a reduction in longitudinal chromatic aberration.
Another object of the invention is to provide a projection lens of a projection display apparatus which not only has a wide field of view of 60xc2x0 or more, but also meets a requirement of a back focal length to focal length ratio (BFL/F) more than 2.8 (BFL/F greater than 2.8) to have a sufficiently long back focal length while having a telecentric structure for allowing the main beam of each field to be incident vertically onto the surface, even in the outermost field, a high-resolution power corresponding to 40% or more of the modulation transfer function (MTF) at a Nyquist frequency determined by the minimum number of pixels in the light valve, a distortion aberration of 1% or less, a low chromatic magnification, and a marginal light amount ratio of 85% or more to provide a uniform brightness over the entire portion of the screen.
In accordance with the present invention, these objects are accomplished by providing in a projection display apparatus including a light source, an optical illumination system, a light valve, a color separating/synthesizing system, and a projection lens, the projection lens comprising:
a first lens group arranged upstream from a screen, the first lens group having a negative power and including at least one aspherical lens element and at least three spherical lens elements, the lens element of the first lens group spaced away from the screen by a maximum distance having a positive power;
a second lens group arranged upstream from the first lens group, the second lens group having a positive power and including a triple cemented lens composed of three lens elements cemented together, and at least one lens element having a positive power arranged at one side of the triple cemented lens opposite to the screen; and
an aperture stop arranged between the first and second lens groups,
the first and second lens groups meet the following conditions:
xe2x88x925.4 less than d/f1 less than xe2x88x920.2xe2x80x83xe2x80x83(1)
0.4 less than d/f2 less than 5.1xe2x80x83xe2x80x83(2)
2.8 less than bf1/f less than 7.8xe2x80x83xe2x80x83(3)
where, xe2x80x9cf1xe2x80x9d represents the effective focal length of the first lens group, xe2x80x9cf2xe2x80x9d represents the effective focal length of the second lens group, xe2x80x9cfxe2x80x9d represents the effective focal length of the projection lens, xe2x80x9cbf1xe2x80x9d represents the back focal length of the projection lens, and xe2x80x9cdxe2x80x9d represents the distance between the first and second lens groups.